1. Field
Apparatuses consistent with exemplary embodiments relate to photoconductive antennas, and more particularly, to photoconductive antennas capable of reducing electromagnetic wave loss.
2. Description of the Related Art
A terahertz frequency band between a micro frequency band and an optical frequency band is a very meaningful frequency band in the fields of molecular optics, biophysics, medical, spectroscopy, imaging, and security. That is, electromagnetic waves of the terahertz frequency band have a difference in absorption characteristics according to properties of materials and have both the directivity of light and the permeability of electronic waves, and thus according to the use of the above features of the terahertz frequency band, the terahertz frequency band may be applied to fields of material analysis, security, medical scanning, and the like.
A photoconductive antenna (PCA) may be used for emission and detection of terahertz (or sub-millimeter) electromagnetic waves. For example, a terahertz PCA may have a structure having a dipole antenna pattern formed on a III-V-group semiconductor substrate formed of gallium arsenide (GaAs) or the like. When an ultrashort pulse laser beam is incident on the dipole antenna pattern, carriers, such as electrons, are excited on the dipole antenna pattern, and the electrons move by an electric field between the two electrodes of the antenna. Electrons generated due to the ultrashort pulses are very quickly generated and extinguished in the substrate, and this phenomenon mainly occurs in a frequency band of about 0.3 THz to about 3 THz. Generated electromagnetic waves pass through the substrate in a vertical direction and are propagated to the air.
According to the photoconductive principle, when a laser beam having greater energy than band-gap energy of a semiconductor substrate is incident on the substrate, electrons are excited to a conductive band and return to a stable state again, and this energy difference, which corresponds to electromagnetic wave energy of a terahertz frequency band in millimeter waves, is called a terahertz photoconductive effect.
In a general PCA, electromagnetic waves emitted from an antenna pattern may travel as surface waves, thereby causing loss, and in addition, electromagnetic waves, which have returned by being reflected from a surface of the substrate opposite the surface on which the antenna pattern is formed, may be refracted by the surface on which the antenna pattern is formed and may be emitted to the outside, thereby causing loss.